The present invention relates to fiber-based adsorbents for the recovery of uranium and other dissolved metals from aqueous solutions.
Uranium is dissolved in seawater across the world's oceans at a uniform concentration of approximately 3.2 ppb with the total amount of uranium in seawater of approximately 4-5 billion metric tons, which is about 1000 times larger than the amount of uranium in terrestrial ores. Most of the dissolved uranium in seawater exists as uranyl tricarbonate ion (UO2 (CO3)34−). Development of uranium adsorbents has been researched since the 1960s including work on hydrous titanium oxide and other metal oxides, however the adsorption capacity (about 0.1 g-Uranium/kg-adsorbent) and mechanical strength of these materials were deemed too low for practical use. In the 1980s efforts shifted towards developing uranium adsorbents containing organic materials including the amidoxime group which was found to be particularly promising for complexing uranyl ions in seawater. Polymeric beads containing these amidoxime groups were initially evaluated, however this approach was abandoned due to practical handling issues.
Fiber-based adsorbents containing amidoxime groups have been researched since the 1980s. Early versions were based on polyacrylonitrile fibers which were reacted with hydroxylamine to form amidoxime groups, however since these groups were formed evenly in the fiber, the mechanical strength of the fiber was insufficient to survive in the sea. To alleviate this issue, graft co-polymerization of polyolefin fibers (e.g., polyethylene and polypropylene) with polymerizable monomers was used to produce either nonwoven or continuous fiber braided adsorbents. This process involved co-grafting nitrile groups (e.g., acrylonitrile) and hydrophilic groups (e.g., methacrylic acid) onto previously-irradiated polyolefin fibers having a diameter of at least 15 microns to form grafted side chains, then reacting the nitrile groups with hydroxylamine (NH2OH) to convert them to amidoxime groups followed by alkaline (e.g., KOH) conditioning.
The nonwoven adsorbents were investigated for many years; however these materials are constructed using short, discontinuous, thermally spun-bonded fibers which have relatively poor mechanical strength compared to continuous fiber forms. In particular, nonwoven adsorbents were evaluated in several seawater experiments and demonstrated uranium adsorption capacities of about 1.5 g-Uranium (U)/kg-adsorbent after 30 days immersion in seawater. Due to their low mechanical strength, the nonwoven adsorbents necessitated their incorporation into large sandwich stacks composed of spacer nets and stack holders placed on large, heavy floating frames which eventually proved too costly for implementation. In addition, the sandwich stacks containing the nonwoven adsorbent prevented good accessibility to the seawater resulting in lower adsorption capacities compared to braided adsorbents. These braided adsorbents are composed of continuous polyethylene fibers that are braided around a porous polypropylene float that can be made into long lengths. The braided adsorbent is currently the material of choice for uranium adsorbents due to the favorable balance of properties including high mechanical strength, elongation-to-break, durability, low cost, chemical resistance (i.e., acids, bases, solvents), as well as their ease of placement and retrieval from the sea. However, the uranium adsorption capacity of the braided polyethylene adsorbents is relatively low, at 1.5 g-U/kg-adsorbent after 30 days immersion in seawater, to be cost effective for implementation.
Accordingly, there remains a continued need for an improved fiber-based adsorbent having an increased adsorption capacity for the recovery of uranium and/or other dissolved metals from seawater, river water and other aqueous solutions.